Device including color separation prism

ABSTRACT

The invention relates to a device including a color separation prism, comprising at least two image sensors whose sensor surfaces are substantially of the shape of a rectangular parallelogram, and a color separation prism whose front surface of a functional part is planar and substantially of the shape of a rectangular parallelogram, whereby the front surface of the functional part of the color separation prism has a first side length and a second side length perpendicular thereto. In the device of the invention, the first side length of the front surface of the functional part of the color separation prism is substantially shorter than the second side length.

BACKGROUND OF THE INVENTION

The invention relates to devices equipped with a colour prism, such ascolour line scan cameras, colour scanners and other such optoelectronicdevices.

Colour line scan cameras are used in industry and in officialapplications necessitating a high frame rate, high-quality colourseparation and a high-resolution image. Colour line scan cameras employtwo basic technologies for producing colour separation. In the firstmethod, different wavelength ranges are provided with separate sensorswhose shape and order vary but which are characterized in that colourseparation is carried out in the actual sensor element, typically usingfilters. In devices according to the first method, sensors of differentwavelength ranges are usually positioned in closely-spaced rows of theirown. A typical device according to the first method is provided withadjacent rows of sensors for blue, green and red colour channels. Thesecond method is based on a colour separation prism which, utilizingselectively reflecting surfaces and total reflection, separates eachcolour onto an image surface of its own. Line image sensors placed onthese image surfaces allow to be located on top of one anotheroptically.

The problem with the devices according to the above-described firstmethod, wherein colour separation is carried out in the actual sensorelement, is inaccurate colour registration since images of differentcolours are not stored at exactly the same point in time from the samesource. A further problem is that images of different colours are notstored from the same direction nor at the same optical distance.

A colour line scan camera equipped with a colour prism enables an imageof each colour of a certain object to be stored at the same moment, fromthe same direction and at the same optical distance, but since in thismethod in addition to air, glass of the prism is also located between anobjective lens and an image sensor, optical characteristics deteriorateby a factor which depends on the length of a distance travelled by lightin the glass. The colour separation prism deteriorates the opticalcharacteristics of a conventional objective lens because conventionalobjective lenses are designed assuming that the medium in front of theimage surface is air. Attempts have been made to solve the problem byproviding colour line scan cameras with objective lenses to enablecharacteristics deteriorating the image quality of the prism to bereduced. This method enables quite good results to be achieved when anobjective lens is designed for a particular application and when thef-number of the objective lens is allowed to deteriorate. A problem withthis method is that the custom-made objective lens makes the colour linescan camera much more expensive. The colour line scan camera is often apart of a more comprehensive computer vision system, in which case usingthe above-disclosed method also makes the entire computer vision systemmore expensive since a deteriorated f-number requires more efficientlightning.

In addition to colour line scan cameras, the above-described prior artproblems also relate to all other corresponding opto-electronic devices,such as colour scanners.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to provide a device which forms aline-like image and which is equipped with a colour separation prism,wherein a distance travelled by light in the material of the prism isshorter than that in known corresponding devices, and whose imagequality is better due to the shorter distance for light to travel. Theobject of the invention is achieved by a device which is characterizedby what has been disclosed in the independent claim. Preferredembodiments of the invention are disclosed in the dependent claims.

The idea underlying the invention is that the distance travelled bylight in the material of a colour separation prism is reduced by shapingthe prism in a novel manner. Advantages of the device of the inventionwhich forms a line-like image include better optical characteristics ascompared to those of a known device which forms a line-like image butincludes no optics for reducing harmful effects caused by a prism, and alower price as compared to that of a known device which forms aline-like image and includes optics for reducing harmful effects causedby a prism.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in closer detail with reference to theaccompanying drawings, in which

FIG. 1 is a cross-sectional view showing a colour separation prism of adevice according to an embodiment of the invention and image sensorsfastened thereto in a plane in the direction of which a blue, a greenand a red component of a ray of light hitting the prism in a directionof an optical axis progress in the material of the prism;

FIG. 2 shows the colour separation prism of FIG. 1 as seen in adirection perpendicular to the direction of view and the optical axis ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 describes a situation wherein a prism 1 according to anembodiment of the invention is used for separating light hitting theparticular prism into a blue, red and a green component. Blue light isconveyed to a blue light image sensor 13 by means of reflection takingplace from a surface 3 reflecting the blue light and total reflectiontaking place from a front surface 2 of the prism. Red light is conveyedto a red light image sensor 12 by means of reflection taking place froma surface 7 reflecting the red light and total reflection caused by anair gap 5. Green light goes straight through the prism 1 to a greenlight image sensor 14.

The device of the invention comprises two or more image sensors 12, 13and 14, whose sensor surfaces are substantially of the shape of arectangular parallelogram, and a colour separation prism 1. A frontsurface 2 of a functional part of the colour separation prism 1, i.e.the surface first hit by the light arriving at the prism 1, is planarand substantially of the shape of a rectangular parallelogram, in whichcase the front surface 2 of the functional part of the colour separationprism has a first side length l₁ and a second side length l₂perpendicular thereto. The device of the invention is characterized inthat the first side length l₁ of the front surface of the functionalpart of the colour separation prism is substantially shorter than thesecond side length l₂.

Since the colour separation prism is preferably shaped such that thecomponents of light hitting the prism to be conveyed to different imagesensors, the components thus being separated using the colour separationprism, travel a distance of equal length in the material of the prism,the shaping in accordance with the invention of the front surface of thefunctional part of the prism affects other dimensions of the prism aswell. In the colour separation prism of FIGS. 1 and 2, for example,which divides the incoming light into three components, the shaping inaccordance with the invention of the front surface 2 of the functionalpart of the colour separation prism 1, wherein the first side length l₁of the front surface is thus substantially shorter than the second sidelength l₂, affects such that a dimension l₃ in depth of the prism alsodecreases as compared to a prior art solution. The distance travelled bylight in the material of the prism 1 decreases by the same factor asused for decreasing the width l₁ of the front surface 2 of thefunctional part of the prism, i.e. by decreasing, for example, the widthof the front surface of the functional part of the prism by half alsoenables the distance travelled by light in the material of the prism tobe halved, etc.

Known prisms are designed for area sensors; therefore, they are largealso in the transverse direction of line image sensors. Using the knownprisms in devices which form a line-like image causes the light hittingthe image sensors of a device to travel an unnecessarily long distancein the material of the prism, which means that the quality of the imagedeteriorates as compared to the solution of the invention.

The reason why devices forming a line-like image enable a novel prism tobe used lies in the way in which light travels from a lens to anelongated image sensor. At the lens, light information supplied to thesensor travels along a path whose cross-section is of a similar shape tothat of the lens, i.e. the cross-section of the above-mentioned path fora normal round lens, for example, is circular. The cross-section of thepath used by the rays of light hitting a particular image sensor reducesas the light travels from the lens to the image sensor such that thecross-section is eventually exactly of the shape of the image sensor,i.e. line-like, being substantially elliptical between the lens and theimage sensor. The present invention utilizes the fact that the closerthe image sensor, the narrower the area necessitated by the colourseparation prism.

In the device of the invention, a ratio l₁:l₂ of the sides of the frontsurface 2 of the functional part of the colour separation prism 1 may bee.g. 1:5, 1:10, 1:20 or 1:30. In the colour line scan cameras accordingto the present invention, for example, the ratio of the sides of thefront surface of the functional part of the colour separation prism willprobably often be about 1:20.

No accurate minimum or maximum limit can be determined for the ratio ofthe sides of the front surface 2 of the functional part of the colourseparation prism 1 in accordance with the device of the invention.However, it is obvious that advantages achieved by the structure of theinvention diminish when the ratio of the sides draws too near to thecorresponding ratio of known colour separation prisms. Similarly, it isobvious that the colour separation prism 1 cannot be made extremelynarrow but matters relating to production and material technologies setcertain restrictions.

It is obvious that in addition to the distance travelled by light in thematerial of the colour separation prism 1, the quality of image to beachieved is also affected by the size of image elements of the imagesensors 12, 13 and 14 to be used. This means e.g. that when imagesensors having a certain image element size are used, a limit value canbe calculated for the “thickness” of the prism, and selecting a prismthinner than that no longer improves the image quality in practice.

In prior art devices, each image sensor 12, 13 and 14 is usuallyenclosed in a separate housing 15 of its own, the housing having a widthl₅ in a direction parallel to a sensor surface, and a depth l₆perpendicular to a part of the prism to which the sensor surface isconnected, in which case the width l₅ is larger than a width l₄ of thesensor surface. The width l₄ of the sensor surface herein refers to alength of the side of the sensor surface in the direction of whichobjects having a direction parallel to the width l₁ of the front surfaceof the prism become produced onto the sensor surface. The width l₅ ofthe housing of the image sensor is parallel to the width l₄ of thesensor surface.

In the prior art devices, the above-described solution wherein each ofthe image sensors 12, 13 and 14 is enclosed separately works perfectlywell. On certain conditions, the solution may also be used in devicesaccording to the present invention. However, in situations wherein acolour separation prism 1 whose front surface 2 of the functional partis extremely elongated is to be used in a device of a relatively smallsize, a situation sometimes occurs wherein the dimensions of thehousings 15 of the image sensors 12, 13 and 14 restrict selection of theratios of the sides of the front surface of the functional part of theprism. In the case of the colour separation prism 1 according to FIG. 1,equipped with three image sensors, such a situation occurs when thedimensions of the prism are reduced to such an extent that the edges ofthe housings 15 of the image sensors 12 and 13 come into contact with aside in a direction parallel with the direction l₃ in depth of the prism1, or the housing 15 of the image sensor 14.

In order for the dimensions of the housings 15 of the image sensors 12,13 and 14 not to restrict the dimensioning of the front surface 2 of thefunctional part of the prism 1, in some situations it is preferable touse a solution according to an embodiment of the present inventionwherein two or more of the image sensors of a device are encased intothe same housing. In many cases it is appropriate to encase all imagesensors of the device into the same housing but it is also possible touse a solution wherein some of the image sensors of the device areencased separately while some are located in the same housing, togetherwith one or more other image sensors. The image sensors may also bepartly or completely non-encased. It is also possible to encase theimage sensors into the same housing as the prism, which provides anextremely compact structure.

Depending on the purpose of use, the device of the invention may includetwo, three or more image sensors. The wavelength of light indicated bythe image sensors used is by no means restricted to the conventionalblue, green and red, nor even to visible light, i.e. when desired, thedevice of the invention may also be arranged for use within a wavelengthrange located partly or completely outside the visible light. It isfurther to be noted that the prism 1 to be used in the device of theinvention does not have to be made of a glass material but any othermaterial having suitable optical characteristics may be used.

For the sake of simplicity, it is assumed in the above description aswell as in the accompanying drawings that the widths l₅ and the depthsl₆ of the housings 15 of the image sensors 12, 13 and 14 as well as thewidths l₄ of the sensor surfaces of the image sensors are the same inall image sensors of the same device. It is, however, completelypossible to construct a device according to the present inventionwherein the dimensions of the housings of different image sensors andthe dimensions of different sensor surfaces differ from each other. Thedimensions of the housings 15 in particular do not have to be mutuallyidentical for different image sensors 12, 13 and 14 although, forpractical reasons, this usually seems to be the case in solutionswherein each image sensor 12, 13 and 14 is encased separately. Neitherdo the dimensions of the sensor surfaces of the image sensorsnecessarily have to be mutually identical although this usually is thecase due to the requirements set by the image shape produced by thedevice for the shape of the sensor surfaces to be used and due to thefact that the components of different colour of light are usually totravel a distance of an equal length in the material of the prism.

Although certain shapes of the components, such as the front surface 2of the functional part of the colour prism 1 and the sensor surfaces ofthe image sensors 12, 13 and 14, have been discussed above by assumingthat they are rectangular parallelograms in shape, it is obvious thatthe present invention also covers solutions wherein the particularshapes slightly differ from that of a rectangular parallelogram. Theinvention thus covers e.g. devices wherein corners of the sensor surfaceof one or more image sensors or the front surface of the functional partof the prism have been rounded, as well as other correspondingvariations.

It is obvious to one skilled in the art that the basic idea of theinvention can be implemented in many different ways. The invention andits embodiments are thus not restricted to the above examples but theymay vary within the scope of the claims.

1. A device including a colour separation prism, comprising at least twoimage sensors whose sensor surfaces are substantially of the shape of arectangular parallelogram, and a colour separation prism whose frontsurface of a functional part is planar and substantially of the shape ofa rectangular parallelogram, whereby the front surface of the functionalpart of the colour separation prism has a first side length and a secondside length perpendicular thereto, wherein the first side length of thefront surface of the functional part of the colour separation prism issubstantially shorter than the second side length.
 2. A device asclaimed in claim 1, wherein a ratio of the first side length of thefront surface of the functional part of the colour separation prism tothe second side length is at least 1:5.
 3. A device as claimed in claim2, wherein it is arranged to operate at wavelengths of visible light. 4.A device as claimed in claim 3, wherein it comprises three imagesensors, the first being arranged to indicate blue light, the secondbeing arranged to indicate green light and the third being arranged toindicate red light.
 5. A device as claimed in claim 1, wherein it isarranged to operate at wavelengths of visible light.
 6. A device asclaimed in claim 5, wherein it comprises three image sensors, the firstbeing arranged to indicate blue light, the second being arranged toindicate green light and the third being arranged to indicate red light.7. A device as claimed in claim 1, wherein the colour separation prismis made of a glass material.
 8. A device as claimed in claim 2, furthercomprising a housing into which the colour separation prism and the atleast two image sensors have been integrated.
 9. A device as claimed inclaim 2, wherein the ratio of the first side length of the front surfaceof the functional part of the colour separation prism to the second sidelength is about 1:20.
 10. A device as claimed in claim 9, wherein it isarranged to operate at wavelengths of visible light.
 11. A device asclaimed in claim 9, further comprising a housing into which the colourseparation prism and the at least two image sensors have beenintegrated.
 12. A device as claimed in claim 10, wherein it comprisesthree image sensors, the first being arranged to indicate blue light,the second being arranged to indicate green light and the third beingarranged to indicate red light.
 13. A device as claimed in claim 1,further comprising a housing into which the colour separation prism andthe at least two image sensors have been integrated.
 14. A device asclaimed in claim 13, wherein it is arranged to operate at wavelengths ofvisible light.
 15. A device as claimed in claim 14, wherein it comprisesthree image sensors, the first being arranged to indicate blue light,the second being arranged to indicate green light and the third beingarranged to indicate red light.